In general, a lithographic printing plate is formed from a lipophilic image area for accepting ink and a hydrophilic non-image area for accepting dampening water in a printing process. Lithographic printing is a printing process in which the property of water and oil-based ink repelling each other is utilized so as to cause a difference in ink attachment on the surface of a lithographic printing plate with a lipophilic image area of the lithographic printing plate as an ink-accepting area and a hydrophilic non-image area as a dampening water-accepting area (non-ink-accepting area), and after inking only the image area ink is transferred to a printing substrate such as paper.
In order to make this lithographic printing plate, a lithographic printing plate precursor (PS plate) formed by providing a lipophilic photosensitive resin layer (photosensitive layer) on a hydrophilic support is widely used in the art. A lithographic printing plate is usually obtained by a process in which, after the lithographic printing plate precursor is exposed through an original image such as a lith film, a photosensitive layer that becomes an image area is made to remain, and unnecessary photosensitive layer that becomes a non-image area is removed by dissolving using a strong alkaline developer having a pH of no less than 12 to thus form a non-image area in which the surface of the hydrophilic support is exposed.
In the conventionally known process for making a lithographic printing plate, after the exposure, a step of removing unnecessary photosensitive layer by dissolving, for example, with a developer is required, and simplification of such wet treatment carried out additionally is cited as one of issues. In particular, since in recent years the disposal of liquid waste discharged accompanying highly pH alkaline treatment has become a great concern throughout the industrial world from the viewpoint of consideration for the global environment, developing by a nearly neutral aqueous solution has been more and more desired as one of the simplification.
On the other hand, digitization techniques involving electronically processing, storing, and outputting image information by computer have been widespread in recent years, and various new image output methods responding to such digitization techniques have been put into practical use. Accompanying this, a computer-to-plate (CTP) technique has been attracting attention in which digitized image information is carried on a highly convergent radiant ray such as laser light and a lithographic printing plate precursor is scan-exposed by this light to directly produce a lithographic printing plate without intervention of a lith film. Therefore, obtaining a lithographic printing precursor suitable for these techniques has become one of the important technological challenges.
As a laser light source that can be used in the CTP technique, a solid-state laser such as a YAG laser or a semiconductor laser that emits infrared rays having a wavelength of 760 to 1,200 nm is very useful since one with high output and small size is available at low cost. A UV laser can also be used.
From the background described above, there is currently an even stronger desire than ever for compatibility with both digitization and simplification of plate making operations.
In response to this, for example, Patent Document 1 describes a process for making a plate by developing, using a gumming liquid, a lithographic printing plate precursor having provided above a hydrophilic support a photosensitive layer in which hydrophobic thermoplastic polymer particles are dispersed in a hydrophilic binder.
Patent Document 2 describes a method for processing a lithographic printing plate precursor by imagewise exposing with an infrared laser a lithographic printing plate precursor comprising (i) a hydrophilic support and (ii) a photosensitive layer comprising a radically polymerizable ethylenically unsaturated monomer, a radical polymerization initiator, and an infrared absorbing dye, and then removing an uncured portion of the photosensitive layer using a gumming liquid.
Furthermore, Patent Document 3 describes a method for developing a lithographic printing plate precursor, the method comprising curing a radically polymerizable photosensitive layer by exposure to an infrared laser and removing an unexposed portion using a solvent-containing developer having a pH of less than 12 and comprising benzyl alcohol and an amphoteric surfactant having a specific structure.
Furthermore, Patent Document 4 describes a method for developing a lithographic printing plate precursor, the method comprising curing a radically polymerizable photosensitive layer by exposure to an infrared laser and removing an unexposed portion using a solvent-containing developer comprising no greater than 60 wt % of water, a water-soluble or water-dispersible solvent, and a surfactant having a specific structure.
Furthermore, Patent Document 5 discloses a method for developing a lithographic printing plate precursor in which a radical polymerization-type photosensitive layer is cured by exposure to a UV-IR laser, and an unexposed portion is removed using an aqueous solution having a pH of 2 to 10 and comprising an amphoteric surfactant and an anionic surfactant.    Patent Document 1: EP 1,342,568 B    Patent Document 2: International publication WO 2005/111727    Patent Document 3: International publication WO 2008/027227    Patent Document 4: U.S. 2010/0216067 A1    Patent Document 5: JP-A-2009-086344 (JP-A denotes a Japanese unexamined patent application publication.)